Although methane is abundant, its relative inertness has limited its utility in conversion processes for producing higher-value hydrocarbons. For example, oxidative coupling methods generally involve highly exothermic and potentially hazardous methane combustion reactions, and frequently require expensive oxygen generation facilities and produce large quantities of environmentally sensitive carbon oxides. Non-oxidative methane conversion is equilibrium-limited, and temperatures ≥about 800° C. are needed for methane conversions greater than a few percent.
One way to avoid this difficulty involves converting methane to a mixture comprising carbon monoxide and molecular hydrogen (the mixture being conventionally referred to as “syngas”), converting the syngas to a mixture of oxygenates, and then converting the oxygenates to olefins. See, e.g., U.S. Patent Application Publications Nos. 2005/0107481 A1, 2008/0033218 A1, and 2007/0259972 A1, which disclose aspects of converting syngas to a mixture comprising C1 alcohol and C2 alcohol, and then converting the mixture to a product mixture comprising ethylene and propylene. According to those references, approximately 100% of the mixture's ethanol is selectively converted to ethylene. The mixture's methanol, in contrast, produces (i) ethylene and propylene, in approximately equal amounts, and (ii) a significant amount of by-products. The by-products can include, e.g., one or more of molecular hydrogen, water, alcohols, carboxylic acids, ethers, carbon oxides, ammonia and other nitrogenated compounds, arsines, phosphines, and chlorides. The by-products can also include hydrocarbons, such as one or more of C4 to C30 olefins, acetylene, methyl acetylene, propadiene, butadiene, butyne, and the like, and combinations thereof.
A more flexible process is desired, which can produce ethylene and propylene over a wide range of relative amounts. In order to increase the efficiency of ethylene and propylene recovery from the product mixture, a process is desired which produces fewer by-products than does the conventional process. Advantageously, by-products (if any) should include (i) molecules which can be efficiently separated from the product mixture's ethylene and propylene, such as water, and (ii) relatively high-value hydrocarbon such as C5+ hydrocarbon.